In most process control systems, valve positioning is accomplished through the use of pneumatic actuators. The pneumatic actuators require compressed, clean, dry air for operation. The air transport requires long runs of metal tubing, filters, and continuously operating compressors. Further, the pneumatic system requires two types of motivational forces, electricity to run the compressor and compressed air.
Some fully electrical valve positioning devices have ben provided but are still subject to several disadvantages. Heretofore, synchronous motors, although desirable, have not been applicable to most systems since they require long bursts of AC voltage for their operation. One of the contributing factors which made it necessary to have long AC bursts to operate the synchronous motor was the inherent friction of the actuator device itself. Such bursts were required to overcome that friction and initiate movement of the actuator which is operatively coupled to the valve device. Because of the nature of the long bursts, it has heretofore been impossible to ascertain the precise moment and duration of the motor shaft movement. Thus, position feedback arrangements have been required to monitor valve position. Such feedback systems are complicated and add an unnecessary dimension to the control system and an additional source of error. A means is required to lock the motor rotor when it is not being driven so that when a power failure occurs a preloaded spring can drive the valve open or closed.
Some prior art devices have included stepper motors with a DC power signal being selectively applied to the field windings of the motor. Such systems however require large and expensive DC power supplies for generating the DC power.
Further, most prior art systems have been at least partially analog in their configuration and the simplicity and precision of a digital valve control system has not heretofore been provided.
Some prior art valve drive systems have included a reversible motor but have been subject to inaccuracies when the motor direction is reversed. Many systems in which reversible operation is required include a motor which is electrically driven in only one direction and returned by means of a mechanical spring device. The mechanical return device introduces a new source of error and inprecision vis a vis an all electric, digital system.